# Homework Help: Motorboat relative motion

1. Apr 16, 2012

### jiboom

a motorboat moving at 8km/h relative to the water travels from A to a point B 10km whose bearing from A is 150. it then travels to C,10km from B and due west of B. if there is a current of constant speed 4km/h from north to south,find the 2 courses to be set.

having a bit of trouble with the book answers here.

for first course i have set the relative velocity of boat to water at 150 to the north,
then velocity of Boat is 8 and v water is 4 pointing down.

in my velocity triangle i have v water going up at end of v rel which is at 150 to the vertical,then v b completes triangle.

now in this triangle i work out the angle to be 14.5,15.5,150. so my bearing will be

180-14.5=165.5 but the book answer is 135.5.

but to get this i need an angle of 44.5 in the velocity triangle,but one of those angles is 150...

so either im doing it wrong or book is lying....

for the second course the book answer is 060 but how can this be?

C is due west of B so how can they set a course of 060?the current is only from north to souht,not east to west. i get the 60 in my velocity diagram but then need to add 180 to get bearing of 240.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Apr 17, 2012

### jiboom

any ideas?

3. Apr 19, 2012

### jiboom

cheeky bump/

having a lot of trouble with rel motion agaisnt a current. got all other questions in the exercise but these allude me. my methods must be ok if im getting the non current questions right so only need a nudge in right direction.